This invention relates to a fluidized-bed jet-mill pulverization method, whereby a high-velocity gas or vapor jet exiting from a nozzle is directed at a fluidized bed of granular material. The particles around the jet are accelerated to a velocity where their impact on particles stationary or moving in the opposite direction causes these to break up. A method of this type has been described earlier in the German patent (DE-PS) 5 98 421.
More recent developments in the realm of fluidized-bed jet mills have focussed on improved particle charging of the fluid jets entering the fluidized bed.
What current state-of-the-art methods still need is an improved pulse exchange between the high-velocity, high-kinetic-energy gas or vapor jets and the granular, nearly stationary, low-kinetic-energy material in the fluidized bed.
Typically, a pulse exchange between the gas or vapor jet and the granular material takes place almost exclusively in the envelope section of the gas or vapor jets. The particle velocity perpendicular to the flow direction is not enough to permit penetration into the center of the gas jet. As a result, the high velocities in the core area of the jet remain largely unutilized in the pulverization process.
A first solution to the problem is disclosed in DE 42 43 438 C2 and corresponding U.S. Pat. No. 5,423,490, the disclosure of which is incorporated herein by reference. This involved a better utilization of the energy carried by the jet by increasing the ratio of the material to be pulverized relative to the gas or vapor jets used for the fluidized-bed jet milling process.
According to that solution, the pulse exchange between the gas or vapor jet and the granular material is to be improved by means of flow channels, provided perpendicular to the flow direction of the jet in the low-pulse areas directly behind the point where the jet exits the nozzle, which channels produce a pressure drop from the surrounding area toward the core of the jet, causing the particles of the milling material to be sucked in toward the center of the jet, where they are accelerated to the impact velocity needed for pulverization.
A shortcoming of this earlier process lies in the fact that the particles of the milling material initially display very little kinetic energy and are accelerated only by the high kinetic energy of the gas or vapor jet. In the process there are substantial differences in velocity between the gas or vapor jet and the as yet accelerated particles of the milling material due to the mass inertia of the particles. Consequently, considerable slippage is likely which in turn causes losses in the flow rate due to turbulences. These flow-rate losses negatively affect economical, cost-effective pulverization that would require a minimum amount of energy.
It follows that accelerating the particles together with the fluid is especially efficient. This phenomenon is also utilized in retro-jet fluid-energy mills employing jet pipes. A method along that line is described in DE 36 20 440 A1 where the bulk material to be broken down is introduced in a pressure extractor chamber and is then expanded i.e. pressure-reduced into a feed pipe together with the precompressed carrier gas and accelerated. In that process, each two jet nozzles operate in mutually opposing directions. Particle pulverization is obtained by the mutual crushing of colliding particles. The drawback here is that the milling effect is limited since each particle is exposed to only one single, one-time break-down impact. Many particles are not broken down at all since the jet coming from the opposite direction deflects them from the impact zone in the center of the jet toward the outside, preventing them from colliding with other particles in the area of the jet.
This has been remedied by directing one particle jet against a solid target, thus assuring that each accelerated particle is subjected to a collision. A process employing that method is described in DE 27 38 980 A1. Its drawback again lies in the high rate of wear of the stationary target.